Such a driving circuit is, for example, an integrated driving circuit of the TL6714 type by Infineon Technologies AG, Munich, which is described in the associated data sheet V 1.61 2001-07-13.
The ignition element driven by such a driving circuit is, for example, a pyrotechnic ignitor which initiates further processes for opening an airbag or for tensioning a safety belt. For safety reasons, the demand exists that such an ignitor must be separated from a supply voltage not only by a single switch element but that there must be at least two elements interrupting the current in the load circuit with the ignitor. Driving circuits for such ignitors must be designed for driving the ignitor with a predetermined ignition current which is, for example, within a range of between 1A and 3A for a predetermined on-period which is, for example, within the range of between 1 ms and 5 ms.
Known driving circuits for such ignition elements are constructed in such a manner that the two semiconductor components or semiconductor switches, with which the ignition element is connected in series, are integrated in a common semiconductor chip. In the case of multi-channel driving circuits such as, for example, the TLE6714, which are suitable for simultaneously driving a number of ignition elements, a number of first and second semiconductor switches are integrated on one chip, where one ignition element can be switched in each case between a first and a second semiconductor switch by external connections of the integrated circuit.
This integration of the semiconductor switches on a semiconductor chip leads to the possibility that an inadvertent deployment (IAD) of airbags or belt tighteners can occur in the case of a grave fault on the chip, for example triggered by an uncontrolled influence from the outside. The redundancy of the system, introduced by the two semiconductor switches connected in series with the ignition element, does not exist completely inasmuch as faults on the semiconductor chip which, for example, can lead to an unintended switching-on of a semiconductor switch, in many cases also lead to an unintended switching-on of the second semiconductor switch.
To avoid this disadvantage, it is known to provide two similar integrated driving circuits on one circuit board which allow a “cross-coupled connection” of the ignition elements as is shown in FIG. 1. In FIG. 1, the reference symbols IC10, IC20 designate two identically constructed driving circuits which in each case comprise a first semiconductor switch HS10, HS11 and a second semiconductor switch LS10, LS11, connections of these semiconductor switches in each case being conducted to the outside in order to switch via these connections an ignition element in series with the semiconductor switches HS10, LS10 and HS11, LS11, respectively. In the case of a cross-coupled circuit, it is then provided to use semiconductor switches of different driving circuits for driving an ignition element Z10, Z20. In the example shown, an ignition element Z10 is thus connected between the first semiconductor switch HS11 of the driving circuit IC20 switch HS11 of the driving circuit IC20 and the second semiconductor switch LS10 of the driving circuit IC10. Furthermore, a second ignition element Z20 is connected between the first semiconductor switch HS10 of the driving circuit IC10 and the second semiconductor switch LS11 of the driving circuit IC20. For driving the semiconductor circuit, there are driver circuits DH10, DL10, DH11, DL11, which can also fulfill protection functions for the semiconductor switches, provided in the individual driving circuits IC10, IC20.
The disadvantage of the arrangement shown in FIG. 1 is the comparatively complex wiring on the board, particularly in the case of multi-channel systems in which more than two ignitors are to be driven.
It is the aim of the present invention to provide a reliably operating driving circuit for an ignition element of a passenger protection system which does not have the abovementioned disadvantages.